Wide band coupling for transmission lines



y 1944- D. E. NORGAARD 2,354,537

WIDE BAND COUPLING FOR TRANSMISSION LINES Filed March 1'7, 1943 UTILIZATION DEVICE UTILIZATION DEVICE Inve ntor Donald E.Noraar-d,

" y His Attorney.

DECIBELS FREQUENCY Patented July 25, 1944 UNITED STATES PATENT OFFlC E WIDE COUPLING FOB TRANSMISSION LINES Donald E. Norgaard, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application March 17, 1943, Serial No. 479,412

4 Claims.

ly becomes necessary to employ portable units which must be connected by low impedance transmission lines. These lines are commonly of the concentric conductor type and are made flexible so that they may be handled easily. One method for connecting such a transmission line is to arrange it in the cathode circuit of an electronic amplifier tube so that the terminating resistance of the line is in the cathode circuit; an objection to this connection is that it necessitates the passing of a substantial amount of direct current through the transmission line. When the trans mission line is coupled to the plate circuit of an electronic amplifier tube a series coupling condenser may be provided which prevents the passage of direct current through the line; however, unless a very large condenser is employed, the signal may be attenuated objectionably in the low frequency end of the band of frequencies to be transmitted. The cost and size of the large coupling condenser are too great for the requirements of many types of apparatus. Furthermore,

even with a very large coupling condenser some slight phase shift at low frequencies is unavoidable. In circuits where even this slight phase shift is objectionable it becomes necessary to provide some coupling arrangement which will provide true linearity over the entire band width.

Accordingly, it is an object of my invention to provide an improved coupling arrangement for transmission lines utilized to transmit signals over a wide band of frequencies and which insures linearity of signal transmission throughout the band; 7

It is another object of my invention to provide a circuit for coupling a low impedance transmission line to an electronic amplifier device through a's'eries capacity and which includes an improved arrangement for preventing attenuation of the transmitted signal in the low frequency portion of the band.

- The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 represents an embodiment of my invention, Fig. 2 represents certain characteristics pertainingto the operation of my invention, and Fig. 3 illustrates another embodiment of my invention.

In general, the coupling apparatus shown in the drawing comprises an electronic amplifier provided with an input network for supplying a signal to be amplified and an output network for coupling the amplified signal to a utilization device. The apparatus includes a low impedance transmission line, the terminating resistance of which is connected as an element in one of the networks, and the proportions of the several elements of the one network are selected with respect to certain elements of the other network so that signals are attenuated in the low frequency end of the band in one of the networks and are correspondingly accentuated in the other ofthe networks. The amounts of attenuation and accentuation are selected so that an overall straight line or flat characteristic results and signals throughout the entire band may be transmitted through the apparatus including the line without frequency discrimination. The network coupled to the transmission line includes a series coupling condenser shunted by relatively high resistance, and as a result, no substantial amount of direct current is passed through the transmission line. If desired, a condenser may be provided in series with th resistance to prevent the passage of any direct current through the line, this condenser, of course, being sufilciently large to pass the low frequency components of the signal.

Referring now to the drawing, the coupling apparatus shown in Fig. 1 comprises two thermionic amplifiers including electron discharge devices In and H connected in cascade. Signals to be transmitted and which lie within a predetermined wide band of frequencies are impressed upon the apparatus across input terminals i 2 and I3, and the amplified signals are supplied to a suitable utilization device I4 connected to the output end of a transmission line IS. The transmission line comprises a central conductor l6 and an external concentric grounded shielding conductor l1, and is provided with a suitable terminating resistance i8 across which the utilization device I4 is connected. Between the devices I0 and II is connected an interstage network 20 which may be considered as an input network with respect to the device II. In the output circuit of the device II is connected a network 2| which includes the terminating resistance l8 of the line i5.

The device I is provided with a cathode 22, a grid or control electrode 23, screengrid 24, suppressor grid 25, and anode 25. The device II includes a cathode 28, control grid 29, screen grid 33, suppressor grid 3|, and an anode 32. Anode and screen voltages for both devices are obtained from a suitable direct current source such as a battery 33. The screen grids 24 and 30 are connected to the battery by resistors 34 and 35 respectively and provided with by-pass condensers 36 and 31. The anode 28 is connected to the battery through a resistance comprising portions 49 and 4|, the portion 4| being lay-passed by a condenser 42. The resistances including the portions 49 and 4| in series constitute a shunt resistance in the input network of the device The anode 32 is connected to the battery 33 through a suitable resistance 43 in the output network 2|. Resistances 44 and 45 are connected in the circuits of the cathodes 22 and 28 respectively. These resistances are not provided with by-pass condensers so that the resultant degeneration is constant at all frequencies and produces no frequency discrimination over the band of frequencies of the impressed signals.

Signals impressed across the terminals |2 and I3 are amplified by the device l0 and appear across the shunt resistance 40. 4|, and these signals are coupled to the grid 29 of the device I I through a suitable coupling condenser 41 and a grid leak resistor 48. The device amplifies the signal which appears across resistor 43 and is coupled to the transmission line through a condenser 49. The condenser 49 is shunted by a relatively high resistance 50, and a blocking condenser may be provided in series with resistance 59 if it is desired to prevent any direct current from flowing through the transmission line.

During the operation of the apparatus, the coupling condenser 49 tends to produce attenuation of the signals in the low frequency portion of the band to 'be transmitted. At the same time. it is essential that the signal be transmitted through the line l5 without so-called phase distortion due to changes in the reactance of the condenser 49 with frequency. It has been found that a flat characteristic may be obtained by proper selection of the several components of the input network 20 and the output network 2|. The relative sizes of the resistance portions 49 and 4| and the capacity of the condenser 42 are selected with respect to the values of the resistances I8, 43, and 59 and the condenser 49 so that attenuation of the low frequency portion of the signal in the network 2| is completely compensated by the accentuation produced in the network 20 in order that a fiat characteristic is resistance 59 and condenser 5| in series should be made greater than the period of one cycle of the lowest frequency of the band to be transmitted. In addition, resistance 43 should be much larger than the sum of' resistances 4| and 49; and the plate resistance of device ll should'also be much larger than this sum.

In Fig. 2, I have illustrated the characteristic curves of the networks 2|! and 2| at and 59, respectively. The abscissa of these curves is a logarithmic scale of frequencies and the ordinate is' in decibels, and since decibels are by deflnition logarithmic, both scales are logarithmic. For any frequency the sum of the decibel values and therefore, the product of the voltage values taken from the curves 55 and 53 is the same and provides the desired straight line characteristic.

In the foregoing, the general arrangement and the relative values of the several components of the input and the output networks of the device II have been described. The manner in which the linear frequency characteristic is obtained whereby signals may be coupled to and transmitted through the low impedance line |5will be more clearly understood from the following analysis.

The network 20 is coupled to the device through the condenser 41 and the resistance 43, and this coupling must be chosen so that it passes the lowest frequency within the band to be transmitted without more than a permissible amount of phase shift. Because resistance 43 must be large, this condition is easily achieved in most cases by the use of a reasonably small size of coupling condenser 41. In the analysis below, the phase shift caused by elements 41 and 43 is'neglected, since it may be made negligible in practice.

The impedance of the network 2| may be expressed by the following equation:

' the frequency multiplied by 2w; a nd i repreprovided throughout the band of frequencies to be transmitted. The loss of the transmission line I5 is negligible, and the resistance l8 may be considered as connected directly to the condensers 49 and 5|. In the two networks 20 and 2|, the ratio of the sum of' the resistances l8 and 43 to the resistance 40 is made equal to the ratio of the resistance 59 to the resistance 4| and to the ratio of the capacity of the condenser 42 to the capacity of the condenser 49. When these ratios have been obtained, the characteristic for the circuit becomes flatover a wide band of frequen cies. It should be understood, of course, that the screen grid circuits of the devices In and II are adequately by-passed by the condensers 33 and 31. The time constants of the condenser and resistance 48 in series and the sents the imaginary quantity \/1. This nomenclature will be employed throughout the following LRLL where Inis the current flowing to the plate 32 of device II, and the impedance of condenser II is negligible compared with the resistance ll within the band o'firequencies to be transmitted:

Substituting Zn from Equation 1 The current In may be expressed in termsof Inc, the current flowing in the load circuit of device II, as follows: I

Now, if Ris+R4a=AR4o, where A- is a real constant; Rsc=AR41, and

frequencies, since In is proportional to 'the applied input voltage, Ea, across terminals -l2 and I3.

In many cases a large voltage is available at the terminals l2 and ii of the apparatus on the input side of the transmission line, and in some cases it may be difiicult to provide ahead of device II, as is done in the network 20 of. Fig. 1, correction for the phase shift caused by the network II. In order to obtain a flat characteristic in this case, the circuit shown in Fig. 3 may be employed; this circuit incorporates the compen sating network in the output of an amplifier located at the output end of the transmission line.

.The first amplifier stage illustrated in Fig, 3 and including the transmission line is the same as the second stage shown in Fig. 1 and corresponding parts or circuit components have been designated by the same numerals. The device III together with its associated circuits is'not employed and the input terminals l2 and I 3 are connected to impress the input signal directly on the grid 2! of the device II. The characteristic curve for the first stage of the amplifier inFlg. 3

is, therefore, the same as that of the second stage in Fig. 1, and is indicated by the curve 56 in Fig.

2. The voltage appearing across the terminating. resistance I8 is amplified and coupled toa utilization device 59 through an amplifier including an electron discharge device 60 having an output network 8| coupled to the utilization device through a coupling condenser G2 and across a resistor 83. The amplifying stage including the device 60 and the network BI is connected in the same manner as the first stage Shawn-mm. 1 and including the device Ill and network 20. The device 60 is provided with a cathode 63', an anode N. a control electrode, 65; and a screen grid 66.

I ponents; of the networks 6| are the ratio of the sum of the resistances and 43 to the'resistance 68 is the same as the ratio which contains no term dependent upon frequency. It follows that the signal will be transmitted linearly throughout the required band of Anode and screen voltages are obtained from asuitable source such as a battery 61, the anode It being connected to the battery 61 through a shunt resistance comprising portions 68 and .69 in the network 6|. The portion 69 is shunted by a condenser 10. The screen grid is connected to the battery 61 through a resistor H by-passed by a condenser 12. A resistance 13 is provided between the cathode 63' and the ground and it is of a value such that correct bias is provided for device 80. The values of the resistance portions 6! and 69 and the capacity of the condenser III are chosen with respect to the values of the components of the network 2| so that compensation is provided for the attenuation of loviffrequency currents produced by the network 2|, the resulting characteristic curve being the same as the curve. in Fig. 2. The values of the comchosen so that of the resistance 50 to the resistance 69 and also the same as the ratio of the capacity of the condenser 10 to the capacity of the condenser 49. The coupling condenser 62 and the resistance 63 are chosen so that they do not cause low frequency phase shift within the band of frequencies to be transmitted.

The mathematical analysis of the circuit shown in Fig. 3 is similar to that of the circuit of Fig. 1. The several components of the first stage of, Fig.

'3 may have the same values as those of the second stage of Fig. 1. The values of the circuit components of the second stage associated with the device are chosen to compensate fully for the phase shift in' the network 2| in accordance with the following analysis.

The voltage E01 across the network at the anode 84 may be expressed as follows:

i la

L R wC where In is the plate current of device 60. This is a correct expression for Eu because the resistance i3 and the plate resistance of the device 89 are both very much larger than the sum of the resistances 68 and 69. Now:

where K: is a real constant of proportionality. Substituting in Equation 9 E51=KgE g R53 -Je- Substituting the value of En from Equation 4 R 8R En= z zi 1 jRw 43+ l8 Rio m Egg-IE Now if R1e+R4a=BRsa, where B is a real constant; Rso=BRee, and

C C49: I?

then substituting in Equation 12 yields R ER n= z a l g BR., l BR, w C1 J R8 (.OCTOj Kzllgg gRu Rflrm But I1e=K1E29, where K1 is a real constant, and E20 is the voltage across the input terminals l2 and I3 of Fig. 3. Substituting:

Equation 14 contains no term which is dependent upon frequency,'. and it follows that linear transmission of the signal is attained throughout the desired frequency band.

0 From the foregoing, it will be evident that my invention provides a transmission line couplin apparatus suitable for the linear transmission of signals over a wide band of frequencies. By way of illustration only and not by way of limitation, there are listed below values of circuit constants which have been found suitable for the circuits of Figs. 1 and 3. The devices l0 and 60 were type 68.17 pentode or triple-grid detector tubes, and the device I I was a type 6AG'1 pentode operated with a plate current of 30 milliamperes. The transmission line had a characteristic impedance of '75 ohms and the lowest frequency transmitted was about 100 cycles per second and the highest frequency was of the order of onehalf megaeycle. The anode or plate supply voltage was 300 volts. The other values follow:

If the condenser 5| is omitted from the network 2l, a direct current of about 2.8 milliamperes will flow through the resistance It when the above values of the circuit components are employed. This direct current is less than 10 per cent of the average plate current of device II and is, therefore, less than 10 per cent of the peak to peak signal capability of the device ll. When the condenser 5| is omitted from the circuit of Fig. 3, the direct current flowing in resistance l8 ill cause the control grid 65 of the device to have a slightly positive average voltage. This may be compensated by increasing resistance 13 to a slightly higher value so that the bias voltage effective on the device 60 is correct for proper operation. Because of the regulating properties of the cathode biased amplifier stage employing device 60, the change of bias caused by omitting the condenser 5| without changing the resistance 13 will be approximately one-tenth of a volt. Except for extremely exacting conditions of operation, this small difference in bias would be of no consequence in a practical application.

The circuit arrangement of Fig. 3 can be employed to transmit input signals extending down to and including zero cycles per second by omitting the condenser 51 and connecting the utilization device 59 directly to the Junction of the anode 64 and resistance 88. The condenser 02 and resistance 63 may then be eliminated. Furthermore, when transmitting very low frequencies and direct current signals, it is necessary that the screen grid potentials be maintained constant; this being accomplished by shunting the condensers 31 and 12 with suitable regulating glow tubes indicated at 15 and I6, respectively. Similar changes obviously may be made in the circuit of Fig. 1. However, it should be noted that the coupling between device l0 and the control grid 29 of the device l'l must be some suitable form of direct current coupling to prevent destruction of the grid 29 by the impressing of the plate voltage on the control grid 29.

The circuit of Fig. 1 has the advantage that no correcting or compensating network is required at the terminating end of the transmission line. Furthermore, stray disturbances are 0 less apt to be picked up by the transmission line because the frequency components of the signal are present in the transmission line in their true magnitudes.

The circuit arrangement of Fig. 3 has the advantage that there is no greater danger of overloading either the device H or the device 60 at low frequencies than there is at high frequencies. Furthermore, the arrangement of Fig. 3 lends itself more readily to direct current coupling than does the circuit of Fig. 1.

While both circuit arrangements provide linear transmission of signals over a wide band of frequencies including very low frequencies, one circuit or the other may be better for a particular application. It is, therefore, apparent that through a transmission line and one which is ap plicable to a wide range of uses.

While I have shown particular embodiments of that I do not wish to be limited thereto since different modifications may be made both in the circuit arrangement and in the instrumentalities employed, and I intend by the appended claims to cover all modifications which fall within the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A transmission line coupling apparatus for transferring alternating current wave energy within a wide band of frequencies comprising a low impedance transmission line having a terminating resistance, an electronic amplifier having an input network and an output network, one of said networks comprising a first shunt resistance and a capacity shunting a portion of said shunt resistance; the other of said networks comprising a shunt resistance, said terminating resistance, and a coupling capacity shunted by a second resistance and connected between said line and said shunt resistance, said first shunt resistance, said portion thereof and said shunt capacity comprising said one network |being proportioned to accentuate current of low frequency within said band, and said coupling capacity and resistances of said other network being proportioned relative to said shunt capacity and said resistance and said portion comprising said one network to attenuate said low frequency current to produce linear transmission at all frequencies in said band.

2. A transmission line coupling apparatus for transferring alternating wave energy linearly within a wide band of frequencies comprising a low impedance transmission line having a terminating resistance, an electronic amplifier having an input network and an output network, one of said networks comprising a first shunt resistance and a capacity shunting a portion of said shunt resistance, the other of said networks comprising a coupling capacity shunted by a second resistance and connected in series with said transmission line and a third shunt resistance on the input side of said coupling capacity, the ratio of the sum of said third shunt resistance and said terminating resistance to that part of said first shunt resistance not shunted by my invention it will, of course, be understood said capacity being equal to the ratio of said second resistance to said portion of said first resistance and these ratios both being equal to the ratio of said capacity shunting said portion to said coupling capacity.

3. A transmission line coupling apparatus for transferring alternating current wave energy within a wide band of frequencies comprising a low impedance transmission line having a terminating resistance, an electronic amplifier, an input network for said amplifier comprising a shunt resistance and a capacity shunting a portion of said resistance, an output network for coupling said amplifier to said transmission line and comprising a shunt resistance and a coupling capacitor connected between said shunt resistance and said line and shunted by a second resistance, said terminating resistance also constituting a shunt resistance in said network, said input resistance and said portion thereof and said shunt capacity being proportioned to accentuate currents of low frequency within said band, and said coupling capacity, shunt resistance, terminating resistance and second resistance being proportioned relative to said shunt capacity and said input resistance and said portion thereof to attenuate said low frequency currents to produce linear transmission at all frequencies in said band.

4. A transmission line coupling apparatus for transferring alternating current wave energy within a wide band of frequencies comprising a low impedance transmission line and a terminating resistance therefor, an electronic amplifier at the input of said transmission line including a shunt resistance having a coupling capacity shunted by a second resistance, an electronic amplifier connected across said terminating resistance and having an output network comprising a shunt output resistance and a capacity shunting a portion of said output resistance, said coupling capacity, said shunt resistance, said second resistance and said terminating resistance being proportioned relative to said shunt capacity and said output resistance and said portion thereof to attenuate low frequency currents in said band, and said output resistance and said portion thereof and said shunt capacity being proportioned to accentuate said low frequency current to produce linear transmission at all frequencies in said band. 1

DONALD E. N ORGAARD. 

